What is Insulation Coordination

8/12/2019 What is Insulation Coordination

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What is Insulation Coordination?

Insulation Coordination is the process of determining the proper insulation levels of various

components in a power system as well as their arrangements. It is the selection of an

insulation structure that will withstand voltage stresses to which the system, or equipment

will be subjected to, together with the proper surge arrester. The process is determined fromthe known characteristics of voltage surges and the characteristics of surge arresters.

Some common terms that must be known when performing an Insulation

Coordination Study.

1. Basic Impulse Insulation Level (BIL)

This is the reference insulation level expressed as an impulse crest (or peak) voltage with a

standard wave not longer than a 1.2 x 50 microsecond wave.

A 1.2 x 50 microsecond wave means that the impulse takes 1.2 microseconds to reach the

peak and then decays to 50% of the peak in 50 microseconds. (Clickherefor a figure of the

BIL waveform)

2. Withstand Voltage

This is the BIL level that can repeatedly be applied to an equipment without flashover,

disruptive charge or other electrical failure under test conditions.

3. Chopped Wave Insulation Level

This is determined by using impulse waves that are of the same shape as that of the BIL

waveform, with the exception that the wave is chopped after 3 microseconds. Generally, it is

assumed that the Chopped Wave Level is 1.15 times the BIL level for oil filled equipment

such as transformers. However, for dry type equipment, it is assumed that the the Chopped

Wave Level is equal to the BIL level.

4. Critical Flashover Voltage

This is the peak voltage for a 50% probability of flashover or disruptive charge.

5. Impulses Ratio

This is normally used for Flashover or puncture of insulation. It is the ratio of the impulse

peak voltage to the value of the 60 Hz voltage that causes flashover or puncture. Or, it is the

ratio of breakdown voltage at surge frequency to breakdown voltage at normal system

frequency (60 Hz).
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Overvoltages that need to be considered when doing an Insulation

Coordination Study.

There are three types of overvoltages that may occur on a plant:

Internal Overvoltages Switching Surges External Overvoltages

1. Internal Overvoltages

These may usually be short power frequency overvoltages or weakly damped oscillatory

voltages. The main causes of these overvoltages are:

Phase to Earth Faults: Single line to Ground, Double line to Ground, 3 Phase toGround.

Load Rejection. Ferro Resonance. Ferranti Effect.

2. Switching Surges

These surges are of short duration, irregular (or impulse form) and highly damped. The

effects of such overvoltages are of great concern when the transmission voltage is greater

than 300kV. However, below 300kV, some causes of these overvoltages are:

Resonance effects when switching transformer feeders, or cables and overhead lines. Ferro resonance encountered on transformer feeder double circuits, when one circuit

is switched out but the other parallel feeder remains energised.

Line energisation may cause switching surges especially at the remote end of the linethat is being energised.

3. External Overvoltages

Power systems that operate below 145kV (example the T&TEC system) overvoltages due tolightning are of greater concern than the previous two types of overvoltages. Lightning

discharges are usually very short, unidirectional and have a shape similar to the BIL

waveform.

The point of insulation flashover depends on

(i) Geographical position of the lightning stroke

(ii) Magnitude of the stroke

(iii) Rise time of voltage wave

(iv) System insulation levels

(v) System Electrical characteristics

(vi) Local atmospheric or ambient conditions


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Overvoltage Surge Protection

There are two methods of overvoltage protection:

1. Rod or Spark Gaps

These devices are easy and cheap to install and are usually installed in parallel with insulators

between the live equipment terminal and earth. Some disadvantages of these devices include:

When they operate, they cause a short circuit fault, which may cause protection tooperate and isolate the equipment.

The sudden reduction in the voltage during operation causes high stresses on theTransformer interturn insulation.

The breakdown of plant insulation varies with the duration of the overvoltage. At lower voltages, where short distance gaps are used, maloperation may occur due to

debris being deposited on the terminals of the gaps.

2. Surge Arresters

Modern Surge arresters are of the gapless Zinc Oxide type. Previously, Silicon Carbide

arresters were used, but their use has been superceeded by the ZnO arresters, which have a

non-linear resistance characteristic. Thus, it is possible to eliminate the series gaps between

the individual ZnO block making up the arrester.

Selection Procedure for Surge arresters

1. Determine the continuous arrester voltage. This is usually the system rated voltage.

2. Select a rated voltage for the arrester.

3. Determine the normal lightning discharge current. Below 36kV, 5kA rated arresters are

chosen. Otherwise, a 10kA

rated arrester is used.

4. Determine the required long duration discharge capability.

For rated voltage < 36kV, light duty surge arrester may be specified.

For rated voltage between 36kV and 245kV, heavy duty arresters may be specified.

For rated voltage >245kV, long duration discharge capabilities may be specified.

5. Determine the maximum prospective fault current and protection tripping times at the

location of the surge arresterand match with the surge arrester duty.

6. Select the surge arrester having porcelain creepage distance in accordance with the

environmental conditions.

7. Determine the surge arrester protection level and match with standard IEC 99

recommendations.

Some Common ratings associated with surge arresters

1. Rated Voltage


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The power frequency voltage across the arrester must never exceed its rated voltage,

otherwise the arrester may not reseal and may catastrophically fail after absorbing the energy

of the surge.

For effectively earthed system:

Maximum phase to earth voltage = 80% maximum line voltage

2. Rated Current

Arresters are tested with 8/20 microsecond discharge current waves of varying magnitudes.

3. Normal Voltage

Nominal continuous voltage that the arrester can with stand before failing or flashover.

4. BIL

Basic Impulse Insulation Level which is the maximum impulse for a 1.2 x 50 microsecond

waveform.

5. Discharge voltage

When the overvoltage impulse reaches this value, the arrester begins to channel energy to

earth.

This system shows a 132kV incoming feeder, which is connected to a 132/66kV transformer

at the substation. The arrester is place between the feeder and the transformer.

It should be noted that the rating of the arrester is 120kV. Since the system is 132kV, and a

tolerance of 10% is assumed, then the maximum voltage that the system can experience is

132kV * 1.1 = 145200V.

However, the system is effectively earthed (via the arrester) and for an effectively earthed

system, the maximum arrester voltage is 80% of the system voltage. Thus,

Normal Arrester voltage = 145200 * 0.8 = 120000 = 120kV.

If a lightning strike occurs on the incoming feeder, three scenarios are possible.

1st Scenario: Impulse voltage of lightning strike is less than the Discharge Voltageof the Arrester

In this case, the lightning strike impulse never exceeds 350kV. The traveling impulse wave passes into the substation. Since the BIL of the transformer is much greater than the peak voltage magnitude of

the lightning impulse, the transformer is not damaged.


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2nd Scenario: Impulse Voltage of lightning strike is less than BIL of arrester, but

greater than Discharge Voltage

In this case, the lightning strike peak voltage is such that 350kV < Vpeak < 650kV. Assume that the impulse is 600kV as shownhere. As the magnitude of the traveling impulse rises to 350kV, as it moves to the arrester,

discharge begins to take place.

The wave is clipped at 350kV as shownhere. This clipped wave passes to the substation and since the BIL of the transformer is

greater than 350kV, the transformer is not damaged.

The balance of energy (600 - 350 = 250kV worth of energy) is discharged to ground.3rd Scenario: Impulse Voltage of lightning strike is greater than the BIL of arrester

In this case, the lightning strike peak voltage is greater than 650kV.

Since the peak voltage is greater than he BIL of the incoming feeder and the arrester,both are damaged.

The arrester experiences flashover or disruptive charge and is destroyed. Since it is destroyed, and open circuit occurs between the incoming feeder and the

transformer.

None of the lightning impulse therefore reaches the transformer at the substation. The transformer therefore remains undamaged.
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What is Insulation Coordination